The present invention relates to an air flow meter for measuring an air flow rate, and specifically to an air flow meter so configured as to be integrated with a throttle for controlling an air-intake of the internal combustion engine.
Conventionally, as for the air flow meter for measuring the air-intake of the internal combustion engine such as automotive engine, thermal type air flow meters are commonly used because they can detects mass-flow rate directly. The heating resistance used here includes a platinum wire winded around a bobbin and coated with glass and a thin-film resistance formed on the ceramic substrate or a silicon substrate. As for the method for measuring the flow rate, there are several well known methods including a method in which the current supplied in the heating resistance for heating the heating resistance at a constant temperature is measured directly and a method in which temperature detecting resistances are placed at upstream side and downstream side of the heating resistance and a temperature difference between those temperature detecting resistances is measured.
As for the method for controlling the inlet air flow, a method for opening and closing an air intake passage by using a throttle valve. Specifically, electronic control throttle systems for controlling electrically the throttle valve by using a motor are generally used due to its excellent control performance.
There is such a problem that the control accuracy is reduced in case of applying a conventional heating-resistance type air flow meter to four or less cylinder automotive engines and for the lower engine speed operation or the overloaded operation in which the pulsation in the intake air flow arises and its amplitude is large and counter flow is formed partially. In order to solve this problem, there is such a solution that plural heating-resistance type air flow meters are placed at the upstream side of the throttle and the counter flow is detected by measuring the phase signal s from those air flow meters, and the control error may be reduced resultantly as disclosed, for example, in Japanese Patent Publication No. 2855401. In addition, there is another solution, as disclosed in Japanese Patent Application Laid-Open Number 6-288291 (1994) and Japanese Patent Application Laid-Open No. 8-218934 (1996) in which the control error including the pulsation effect in the air flow rate measured by the heat-resistance type air flow meter placed at the upstream of the throttle is compensated by referring to the output from the pressure sensor placed at the downstream of the throttle, and the control error may be reduced resultantly.
In attempting to integrating the throttle apparatus and the air flow meter, there arises another problem in which the measurement error for the air flow rate increases due to the changes in the throttle valve opening. In order to solve this problem, there is such a known solution as disclosed, for example, in Japanese Patent Application Laid-Open No. 9-53482 (1997) in which the measurement error may be reduced by optimizing the configuration and layout of the air flow meter in relative to the throttle valve.
However, there is a first problem that the error due to the backflow is not decreased sufficiently because the error due to the backflow increases in the internal combustion engine using variable valve mechanism, in the methods described in Japanese Patent No.2855401, Japanese Patent Application Laid-Open No.6-288291 and Japanese Patent Application Laid-Open No. 8-218934.
Further, there is a second problem that the measurement error to the change of opening of the throttle valve is not decreased sufficiently in the methods described in Japanese Patent Application Laid-Open No.9-53482.
Further, there is a second problem that the operational state of the throttle device has not been diagnosed when the electronically controlled throttle is used in the conventional air flow meter.
Further, there is a fourth problem that when there are a lot of engine cylinders in the conventional air flow meter, it is not possible to detect directly the mass air flow rate and to detect the air flow rate every cylinder.